Short answer, low impact.
More resistance.

Athletes of all sorts can attain the benefits of deep water running. It is a great form of cross training and an excellent way to deal with and prevent injuries. Water running can allow an injured athlete to recover without loosing fitness. It can also help with improving running form and decreasing risk of sustaining injuries by decreasing the stress on running on hard surfaces. The most popular form of aqua training devices among runners is the AquaJogger. Made from EVA foam it looks like a belt that you strap around your waist. Almost any flotation vest or belt will work nearly as well. The goal is to keep your body afloat while allowing you to perform a running motion without stress to the body.

Submerged in the water you will have resistance on all sides. This forces opposing muscles to work equally. As you move your arms and legs against the resistance of the water you will get a great cardiovascular, as well as, strength training workout. Adding additional hydro devices, such as a ’socks’, dumbbells, and ‘gloves’ allows you to vary your effort and intensity.

In addition to increasing the resistance of movement, being in the water decreases the stress on your joints, while increasing your range of motion. Because there is no impact, an injured athlete can train in the water and remain fit while injuries heals.

An uninjured athlete can use water running as a cross training method, a way to work on technique, increase resistance while reducing mileage and risk of injury. It’s also a way to ease exercise boredom.

The spine of males and females begins to degenerate at age 18. So with age, we need consider the orthopedic consequences of our exercise routines. You may be 16 years old. If so, I would run on a land surface. Summation: you should use aquatic exercises if you are 45 or above. You don’t need to exclude running or speed walking but a mix of the water and land would be appropriate. It really is age dependent.